The present invention relates to a switching power supply that derives a stable DC power supply from an AC input power supply. Specifically, the invention relates to a switching power supply that sets the input voltage and input current fed to a chopper circuit to be essentially in-phase with and similar to each other for improving the power factor.
FIG. 5 is a block circuit diagram of a conventional switching power supply.
Referring now to FIG. 5, the output from AC power supply 1 is fed via filter circuit 2 to rectifier circuit 3 formed of a diode bridge. Rectifier circuit 3 conducts full-wave rectification of the output from AC power supply 1. Capacitor 4 removes high frequency noise from the output from rectifier circuit 3. A current is fed to smoothing capacitor 7 via inductor 5 and diode 6. As the current is fed to smoothing capacitor 7, smoothed DC voltage Vout is outputted. Switching device 8 such as a MOSFET is connected between inductor 5 and diode 6. Switching device 8 controls the on-off of the current that flows from inductor 5 to diode 6.
Output voltage error detector circuit 9 amplifies the error between the signal obtained by dividing output voltage Vout with voltage-dividing resistor 9R and set voltage Vref. Output voltage error detector circuit 9 feeds the error amplified therein to multiplier circuit 10 as output voltage error signal Verr. Input voltage detector circuit 11 detects the input voltage to a chopper circuit formed of capacitor 4, inductor 5, diode 6, and smoothing capacitor 7. In other words, input voltage detector circuit 11 detects the output voltage from rectifier circuit 3 and outputs input voltage signal Vin. Input voltage detector circuit 11 is formed of a voltage-dividing circuit using resistors or a level-shift circuit. Input voltage signal Vin is in-phase with and similar in the waveform thereof to the output voltage from rectifier circuit 3, that is the absolute value of the AC input voltage from AC power supply 1. (If described strictly, the waveform of input voltage signal Vin, from which a DC bias component is subtracted, is similar to the waveform of the output voltage from rectifier circuit 3 in some cases, in which a level-shift circuit is employed for input voltage detector circuit 11.) Multiplier circuit 10 multiplies output voltage error signal Verr and input voltage signal Vin generated in input voltage detector circuit 11. As a result, multiplier circuit 10 generates current control signal Ith in phase with and similar to input voltage signal Vin and having an amplitude proportional to output voltage error signal Verr.
The current that flows through inductor 5 is converted to current detection signal Vi with detecting resistor 13R and current detector circuit 13. Current detection signal Vi is compared with current control signal Ith in comparator circuit 12. The output from comparator circuit 12 is fed to the reset input (R) of flip-flop 14. Flip-flop 14, reset by the output from comparator circuit 12, outputs a signal at a low level from the output terminal (Q) thereof. Oscillator circuit 15 is connected to the set input (S) of flip-flop 14. Oscillator circuit 15 sets flip-flop 14 at a certain switching frequency and changes the output from flip-flop 14 to a signal at a high level. The output from flip-flop 14 is fed to driver circuit 16. Driver circuit 16 turns switching device 8 on, when the input to driver circuit 16 is at the high level. Driver circuit 16 turns switching device 8 off, when the input to driver circuit 16 is at the low level.
As switching device 8 is turned on, the current from inductor 5 increases and current detection signal Vi rises. As current detection signal Vi exceeds current control signal Ith, the output from comparator circuit 12 is set at a high level and the high-level signal is fed to the reset input of flip-flop 14. As a result, the output from flip-flop 14 is set at the low level. The low-level output from flip-flop 14 turns switching device 8 off via driver circuit 16. As switching device 8 is turned off, the current from inductor 5 decreases gradually. However, since flip-flop 14 is set at a certain frequency by oscillator circuit 15, the output from flip-flop 14 changes to a high-level at a time point, at which the current from inductor 5 decreases to some extent, and switching device 8 is turned on via driver circuit 16. Thus, a current in-phase with and similar to input voltage signal Vin flows through inductor 5, resulting in an improved (corrected) power factor.
For reducing the inductor size in the power supply circuit as described above, it is necessary to raise the switching frequency. As the switching frequency becomes higher, the switching loss increases, impairing the conversion efficiency. It is possible to improve the conversion efficiency by lowering the switching frequency. However, for removing the conduction noise caused, it is necessary to set the cutoff frequency of the filter circuit low, requiring an enlarged filter circuit.
As an example of the countermeasures against the problem described above, the following Patent Document 1 discloses a power factor correction circuit that improves the conversion efficiency by controlling the switching frequency in response to the AC power supply voltage and by reducing the switching frequency in the region, in which the AC power supply voltage is low.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2004-282958
However, in the power factor correction circuit disclosed in the Patent Document 1, the switching frequency shows the maximum in the high-AC power-supply-voltage region, in which a large coil current is caused. Therefore, the power factor correction circuit disclosed in the Patent Document 1 is not always very efficient to reduce the switching loss.
In view of the foregoing, it is desirable to obviate the problems described above. It is also desirable to provide a switching power supply circuit that facilitates reducing the switching loss and the conduction noise caused.
Further objects and advantages of the invention will be apparent from the following description of the invention.